In comparison with general soda-lime glasses, a colored glass can lower the visibility in motor vehicles because of its low visible light transmittance (LTa) and has a function of reducing heat absorption into motor vehicles because of its low solar transmittance (Tsol). In addition, since it has a low ultraviolet ray transmittance (Tuv), the damage to fabrics and the discoloration or decomposition of articles inside motor vehicles due to ultraviolet rays can be prevented.
Various elements can be incorporated in a colored glass in order to improve the glass color, UV-blocking effect, and solar ray absorption effect. The representative elements used in a colored glass include iron (Fe), cobalt (Co) and selenium (Se), and manganese (Mn), nickel (Ni), copper (Cu), chrome (Cr), titanium (Ti) or cerium (Ce) may be further used. Each of these elements has unique coloring effects, and absorption characteristics of ultraviolet rays and solar rays. Such characteristics result from the phenomenon that each element absorbs its specific wavelength. Accordingly, the desired color and transmittance may be designed through the combination of added elements in appropriate ratios.
By properly adjusting various elements as explained above, it is possible to prepare a neutral gray or dark, neutral green-gray glass having aesthetic merit, and function of absorbing ultraviolet rays and solar rays. Various compositions of such gray soda-lime glass have been known and can be divided into two groups, in which one group is prepared by using Fe, Co, and Se as basic elements for coloring while the other group uses additional elements.
In a process for preparing a colored glass, after raw materials are fed into a smelting furnace at high temperature, chemical reactions between the raw materials and decompositions thereof occurs. At the initial stage, the reactions take place in half or less of a continuous-type smelting furnace and in the other part, the fining or refining procedure of bubbles and cooling procedure to a temperature for molding glass melt are conducted. In the fining or refining procedure of bubbles, the bubbles generated in a glass-melting procedure move up to the surface and pop, or they are reabsorbed in glass. The effect depends upon the atmosphere, melting temperature and time in the glass and smelting furnace, and the type and amount of refining agent used. The number of bubbles remaining in the glass is a quality item that is strictly controlled in manufacturing all glass products such as plate glass, glass bottles, etc.
To obtain a proper level of remaining bubbles (i.e., a proper melting quality), a proper length of smelting furnace and proper melting time are needed. In terms of the raw material, it is necessary to use a refining agent such as sodium sulfate (Na2SO4) in a sufficient amount. Sodium sulfate (Na2SO4) decomposes and produces a large amount of gas in glass, which grows fine bubbles and accelerates the fining or refining procedure for moving them up to the surface. It is also helpful in homogenizing the glass. However, in decomposition and gasification, sodium sulfate (Na2SO4) unfortunately generates a large amount sulfur oxides (SOx) which are air pollution materials, and the generated vapor of sulfuric acid and fine powder of sodium sulfate cause serious damage to the refractory materials constituting a heat-condensing chamber of the smelting furnace. Thus, various methods have been tried to reduce the generation of sulfur oxides but no satisfactory results have been obtained. Particularly, no method for reducing sulfur oxide generation that is properly applicable in manufacturing a dark, neutral gray and green-gray glass has yet been known.
U.S. Pat. No. 4,873,206 discloses a glass composition comprising, per 100 parts by weight of a base glass composition, 0.6 to 1.0 parts by weight of total Fe2O3, 100 to 200 ppm of CoO and 50 to 200 ppm of Se, and not containing Ni or Cr. However, this composition has a visible light transmittance (LTa) of about 25 to 30% as measured for a reference thickness of 4 mm and thus is not suitable for providing a function of privacy protection which is one of the important functions required for the sunroof or rear privacy glass of motor vehicles. Furthermore, from the above composition, it is difficult to expect the advantage of reducing sulfur oxide generation according to the reduction of the amount of sodium sulfate (Na2SO4) used in the glass manufacturing.
Korean Patent Laid-open Publication No. 1992-0007944 (U.S. Pat. No. 5,393,593) discloses a glass composition comprising, per 100 parts by weight of a base glass composition, 1.0 to 1.7 parts by weight of total Fe2O3, 100 to 200 ppm of CoO and 10 to 50 ppm of Se. However, since this composition has a visible light transmittance (LTa) of about 13 to 27% and an ultraviolet ray transmittance of 2 to 19%, it is not suitable for providing a function of privacy protection which is one of the important functions required for sunroof or rear privacy glass of motor vehicles, and has a defect of low performance in the function of preventing the damage to fabrics and the discoloration or decomposition of articles inside motor vehicles due to its high ultraviolet ray transmittance. Furthermore, from this composition, it is difficult to expect the advantage of reducing sulfur oxide (SOx) generation according to the reduction of the amount of sodium sulfate (Na2SO4) used in the glass manufacturing.
Korean Patent Publication No. 10-0227250 (U.S. Pat. No. 5,352,640) discloses a glass composition comprising, per 100 parts by weight of a base glass composition, 1.4 to 4 parts by weight of total Fe2O3, CoO≧200 ppm, Se≦50 ppm, and CoO+Se+Cr2O3≦0.24 parts by weight. However, Cr in this composition is known as a poorly soluble material, and as its amount increases, non-molten materials are generated and non-uniform color may be caused thereby. Furthermore, Cr is known as a representative green colorant and thus a dark green color is provided by using a very small amount thereof. Accordingly, if this element is used, there is a difficulty in production of glass with high color uniformity. In addition, because of its high ultraviolet ray transmittance, this composition has a defect of low performance in the function, which is required for sunroof or rear privacy glass of motor vehicles, in preventing the damage to fabrics and discoloration or decomposition of articles inside motor vehicles due to ultraviolet rays. Furthermore, from this composition, it is difficult to expect the advantage of reducing sulfur oxide (SOx) generation according to the reduction of the amount of sodium sulfate (Na2SO4) used in the glass manufacturing.
Korean Patent Publication No. 10-0295379 (U.S. Pat. No. 5,308,805) discloses a glass composition comprising, per 100 parts by weight of a base glass composition, 1.3 to 2 parts by weight of total Fe2O3, 200 to 400 ppm of CoO and 2 to 30 ppm of Se, and containing Ni. In case of containing Ni, it may exist as NiS in the glass-melting procedure and NiS is known to cause glass breakage by volume expansion in the glass-tempering procedure. A dark, neutral green-gray soda-lime glass may be used for motor vehicles and a glass for motor vehicles must be tempered. Therefore, the use of Ni may cause a problem of faulty product increase due to glass breakage. Furthermore, from this composition, it is difficult to expect the advantage of reducing sulfur oxide (SOx) generation according to the reduction of the amount of sodium sulfate (Na2SO4) used in the glass manufacturing.